373.
If we cause a polished plated surface to be so acted on by aqua fortis that the copper within is touched, and the surface itself thus rendered rough, and if the sun's image be then reflected from it, the splendour will be reverberated from every minutest prominence, and the surface will appear iridescent. So, if we hold a sheet of black unglazed paper in the sun, and look at it attentively, it will be seen to glisten in its minutest points with the most vivid colours.
374.
All these examples are referable to the same conditions. In the first case the luminous image is reflected from a thin line; in the second probably from sharp edges; in the third from very small points. In all a very powerful and circumscribed light is requisite. For all these appearances of colour again it is necessary that the eye should be at a due distance from the reflecting points.
375.
If these observations are made with the microscope, the appearance will be greatly increased in force and splendour, for we then see the smallest portion of the surfaces, lit by the sun, glittering in these colours of reflection, which, allied to the hues of refraction, now attain their highest degree of brilliancy. In such cases we may observe a vermiform iridescence on the surface of organic bodies, the further description of which will be given hereafter.
376.
Lastly, the colours which are chiefly exhibited in reflection are red and green, whence we may infer that the linear appearance especially consists